Thermal sensors have a wide variety of uses. In some applications such as health or weather applications, a simple mercury thermometer is sufficient, while other cases require the measurement of the temperature or temperature changes with much finer resolutions. For example, imaging applications and in particular radiation sensors (pixels) for imaging in the infrared and sub-millimeter range are particularly demanding in terms of sensitivity, e.g. fractions of degrees. Other applications include but are not limited to temperature monitoring in digital processors and transduction of other physical effects, and in particular Terahertz (THz) imaging.
Some known thermal sensors include thermopiles, diodes, high Temperature Coefficient resistors (TCR), pyro-electric detectors and thermo-mechanical capacitors.
Some applications use thermal devices manufactured in standard CMOS technology. The sensitivity of such sensors is generally limited by the ratio of their output referred noise and their electrical responsivity. The lower this ratio, the higher is the temperature resolution or the accuracy of the sensor. In some implementations, improving the resolution include boosting the responsivity, such as in resistive bolometers operated with very large pulses of current, which amplify the signal. In other implementations, despite very low responsivity in comparison to other devices, a device may also have very low electrical noise, typically due to the lack of bias current flowing during operation, thus providing for high resolution.
A MOSFET-based thermal sensor has high temperature responsivity, similar to that of non-CMOS compatible materials, and it can be operated with significantly lower currents. Low currents allow for continuous operation rather than pulsed operation, low noise bandwidth and prevention of self-heating. CMOS detectors may be integrated in any standard silicon integrated circuit (IC).
Known detectors using MOSFET-based thermal sensors achieve sufficient performance for applications if the signal to be measured is large in comparison to the fundamental noises that are generated in the detector. One such case is IR detectors, since room temperature objects emit relatively high power in the relevant wavelength range.
Some applications of particular interest include the subject of Terahertz (THz) imaging, which is a significant nondestructive evaluation technique, which may be used for dielectric materials analysis and quality control in the pharmaceutical, biomedical, security, materials characterization, and aerospace industries. The use of THz waves for non-destructive evaluation enables inspection of multi-layered structures and can identify abnormalities from foreign material inclusions, disbond and delamination, mechanical impact damage, heat damage, and water or hydraulic fluid ingression.
Such applications, however, may have higher requirements in terms of temperature resolution. For example, using Planck's law and geometrical optics it can be shown that a pixel of a passive Terahertz camera must be able to detect a temperature change which is significantly smaller than the lowest temperature change that is theoretically achievable using the mentioned solutions.
A limitation of MOSFET-based detectors lies in the significant 1/f noise, caused by defects of the silicon and oxide interface. This noise and also the sensor's responsivity, are substantially proportional to the bias current, therefore it is impossible to overcome this limitation by driving a large-pulsed current. 1/f noise could be reduced by increasing the width and/or length of the sensing MOSFET. However a larger MOSFET will create a larger thermal mass and an undesirably slower response. When applied to electro-optical devices, CMOS thermal sensing may benefit from the modulation of the optical signal with a chopper wheel, and the subsequent demodulation of the electrical signal after the transduction process. Optical modulation applied to thermal sensors, however, is not much effective due to the slow thermal time constants that are typically associated with those sensors. The slower the sensor is, the lower is the modulation frequency that can be successfully applied with a chopper. In practice, optical modulation is inefficient whenever the knee frequency of the noise spectrum is above a few kilohertz, which is the typical case for MOSFETs used as temperature sensors.
Thus, existing technologies cannot provide sufficient resolution to high-requirement applications, such as THz imaging.